1. Field of the Invention
This invention pertains to an improved process for acidizing subterranean formations surrounding a wellbore to increase the permeability thereof. This invention also pertains to novel gelled acid compositions useful in said process and to a process of making such gelled acid compositions.
2. Description of the Prior Art
"Acidizing" is a term applied to a variety of processes in which an acid is used to increase the permeability of a subterranean formations penetrated by a wellbore. The acids dissolve one or more components of the subterranean formation and thereby increase the overall permeability of the formation. The types of acids used in the general processes have been summarized in the book "Acidizing Fundamentals," edited by B. B. Williams et al., Monograph Vol. 6, Henry L. Doherty Series, published by the Society of Petroleum Engineers of AIME, New York (1979). The text indicates that the normally used acidizing techniques fall broadly into three categories: acid washing, matrix acidizing, and acid fracturing.
In matrix acidizing, acid is injected into the formation at a pressure below which a fracture is formed. Matrix acidizing treatments generally are thought to give radial acid penetration into the formation. This technique is often used where acid fracturing cannot be risked because of possible damage to a shale break or other natural flow boundry which must be maintained in order to minimize or prevent water or gas production.
Acid fracturing is a generic term for techniques in which the acid is injected into the formation at a pressure high enough to create or extend a fracture into the formation. The acid reacts with the formation as it opens up the fracture and provides a conductive flow channel when the treatment is complete. The length of the fractures created and the degree of fracture conductivity is determined in large part by the rate at which the acid spins (i.e. the rate at which the acid reacts with dissolvable components in the formation) and the rate of fluid loss from the fracture to the formation.
Gelled acids have been used to retard the reaction rate of the acid with the formation in acid fracturing treatments. One theory is that the reduced reaction rates is a result of the increased fluid viscosity which reduces the rate of acid transferred to the fracture wall. Various polymers have been used to create the acid gels. Typically, prior art gelling agents are of limited value because they tend to degrade rapidly in acid solutions at temperatures exceeding about 130.degree. F. As a result of this, such gelled acids are not viable in deep high temperature wells. In addition, conventional acid gelling agents have typically been ineffective in gelling the more concentrated acids, e.g. hydrochloric acid at a concentration of about 20 to about 28 percent by weight.
The gelling agent is described by Parks in U.S. Pat. No. 3,236,305 is probably the most effective gelling agent used to date. Parks teaches that xanthan gum, a biopolymer, is an acid gelling agent and he also teaches how to use the thickened acid in the treatment of subterranean formations. A wide variety of other polymers have been included in acid fluids for well treatments as illustrated by:
U.S. Pat. No. 3,434,971 (Atkins) Copolymers of Acrylamide and N-Vinylpyrrolidone Crosslinked with Difunctionalolefinic Crosslinkers. PA1 U.S. Pat. No. 3,924,684 (Tate) Poly (N-Vinyl pyrrolidone). PA1 U.S. Pat. No. 3,974,077 (Free) Galactomannan Gums and Derivatives Thereof. PA1 U.S. Pat. No. 3,548,945 (Gidley) Low Molecular Weight Aliphatic Glycol Ethers. PA1 U.S. Pat. No. 3,475,334 (Boudreaux) a Dialdehyde Surface Treated Cellulose Ether (e.g. Glyoxal-Treated Cellulose Ethers). PA1 U.S. Pat. No. 4,235,734 (Scherubel) Foamed Acids Stabilized with Alkanols. The term "Alkanols" includes polyoxyalkalene glycols and monoethers thereof. PA1 U.S. Pat. No. 4,237,974 (Scherubel) certain Cellulose Derivatives and Ethylene oxide Polymers used to Thicken Pad Fluids. PA1 U.S. Pat. No. 4,237,975 (Scherubel) Ethylene oxide Polymer Pad Fluids Preceding a Foamed Acid.
U.S. Pat. No. 3,791,446 (Tate) Poly (N-Vinyl pyrrolidone).
The polymers of ethylene oxide, generally referred to as polyoxyethylene glycols, are commercially available products which are obtained by a variety of polymerization processes with different molecular weights. J. J. Stratta taught in a Union Carbide trade bulletin that acids were thickened by certain polyoxyethylene glycols. The bulletin was entitled "POLYOX POLY (ETHYLENE OXIDE) RESINS, THE THICKENING OF ACIDS AND OTHER CLEANING FORMULATIONS," Union Carbide technology series number 45-T-5 (March 1970). Stratta disclosed that strong acids such as hydrochloric acid and nitric acid, are thickened to progressively lower viscosity levels as the concentration of acid is increased and that the formulation viscosity stability decreases with increasing molecular weight.
The higher molecular weight polyoxyethylene glycols are water soluble solids. Unfortunately, however, the higher molecular weight polymers are difficult to dissolve because of their molecular weight and because as they dissolve the aqueous solution thickens so that stirring becomes very difficult.